Pokemon Emerald Question and Answers :

how do you catch Deoxis???

I will say this again:
After you defeat the Elite Four, Go in front of the Moss...

I will say this again:
After you defeat the Elite Four, Go in front of the Mossdeep Space Center. Then save. After that, talk to everyone in the space center. When someone asks you if you want to go to space say yes. You will only have 15 min. and Deoxys will be at lv. 70. (Now do you see why you should save?)